1967 Volume 40 Issue 9 Pages 1999-2003
The PMR spectra of 5 benzoxazole derivatives were measured in CCl4 (or CDCl3) and CF3COOH. It was found that, due to the magnetic anisotropy of heteroatoms and the electricfield effect, ring protons shift to a lower field than had been anticipated from the π-electron densities (in the case of 4-H, ca. 0.4 ppm). In acid media, the π-electron density distribution of benzoxazole is changed because of the protonation of the nitrogen atom, and these two effects vanish. Correcting the measured shifts with Pople’s and Buckingham’s expressions for these effects, the following relation has been found to hold between the corrected shifts and the π-electron densities calculated with the Hückel LCAO MO method:
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